At the start of a telephone call in a long-delay telephone circuit using an echo canceller, since the connection of a telephone circuit has just been completed, the impulse response must be rapidly subjected to a large amount of correction from its initial value. If this correction is delayed, a large residual echo will occur at the start of the telephone call. On the other hand, after the impulse response has been formed to some extent, that is, after an appreciable period of time has elapsed from the start of the telephone call, it is desired that the correction of the impulse response is an operation slow enough to follow-up a circuit condition change and still provide an accurate follow-up response.
However, the conventional echo canceller cannot satisfy in principle the above contradictory requirements.